There has been a great deal of interest in developing better and more efficient methods for storing energy for applications, such as cellular communication, satellites, portable computers, and electric vehicles to name but a few. Accordingly, there have been recent concerted efforts to develop high energy, cost effective batteries having improved performance characteristics and a variety of form factors.
In an effort to lessen the weight of battery cells, such as rechargeable electrochemical cells, while providing more flexible form factors, battery manufacturers have shown increasing interest in lithium ion polymer electrochemical cells. In lithium ion polymer technology, the bonding of the electrodes, (the anode and the cathode) to the separator eliminates the need for rigid cell cans. Accordingly, lithium ion polymer cells may be packaged in thin, flexible, multilayered packaging. This multilayered packaging typically includes at least one layer of a thin metal foil, such as aluminum, to provide an oxygen and moisture barrier. The foil layer is then typically encapsulated between a sheet of a mechanically robust outer polymer material, such as nylon or polyester, and a layer of a low melting temperature polyolefin for heat sealing. These metal foil laminates are very common in everyday usage, having applications in the pharmaceutical and food packaging industries.
There are a number of problems which are prevalent when this type of foil laminate packaging is applied to battery applications. Each problem is related to current conducting metallic tabs which pass from the interior of the cell package to the outside in order to electrically couple the battery cell to an application device. For instance, package delamination commonly occurs as a result of insufficient bonding between package heat sealing layers and the metallic surface of the tabs. Furthermore, insufficient tab thickness commonly results in low tab strength. Restrictions on tab thickness (generally to 2-3 mils) are a product of the foil laminate structure, i.e., tab thickness is restricted to allow the heat seal layer to flow around the tabs to produce a hermetic seal. Finally, unintentional electrical shorting between individual tabs and between the tabs and the metal foil laminate, although not as significant a problem as the aforementioned problems, remains a concern to be addressed.
One novel foil laminate packaging structure for electrochemical cells, disclosed in commonly-assigned U.S. patent application Ser. No. 08/901,858, incorporates a modified polyolefin adhesion-promoting material which provides improved delamination protection and increased tab strength. Despite the associated improvements in delamination resistance and tab strength, these issues remains a concern and there is a constant effort to further improve the design of such foil laminate packages. Therefore, it would be desirable to provide a modified foil laminate structure which further improves the mechanical integrity of the package.
Accordingly, there exists a need to provide a better structure for packaging electrochemical cells, such as lithium ion polymer cells. The packaging scheme should provide a tab structure which provides better tabs strength, reduced delamination, and reduces or eliminates tab shorting.